Introduce svn_sort__array_reverse(), and use it in svn_rangelist_reverse()
which should now be more efficient because it was previously swapping the
pointed-to structures rather than just the pointers.

* subversion/include/svn_sorts.h,
  subversion/libsvn_subr/sorts.c
  (svn_sort__array_reverse): New function.

* subversion/libsvn_subr/mergeinfo.c
  (svn_rangelist_reverse): Use it.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1445967 13f79535-47bb-0310-9956-ffa450edef68